In a radiometric sorting system ore particles are arranged in parallel streams with the particles in each stream separated from each other.
The particles in each stream are passed over a plurality of spaced scintillation detectors and each detector records a radioactive count for each particle as it passes. The counts from the individual detectors pertaining to the same particle are then accumulated to obtain a final determination of the radioactive content of the particle and a particle accept or reject decision is based on this determination.
With large spacings between adjacent particles this method functions adequately but as the spacings decrease the accumulated count derived for a given particle, (P), is influenced by fringing effects arising at least from a preceding particle (P-1), and a following particle (P+1).
Due to the continuous and random nature of the emission of radiation from radioactive material, when the particle (P) is within the gated counting zone of a particular scintillation detector, particles (P-1) and (P+1) are also emitting radiation which is also seen and counted by the detector and associated counting electronics as being due to particle (P). The result is that if either particle (P-1) or (P+1) is of fairly high grade ore, and particle (P) is of waste or low grade ore, particle (P) may have an apparent high count and consequently be incorrectly sorted by the machine as ore, when it is actually waste, the final result being to dilute the accept ore fraction. This effect is unavoidable at the particle to detector distances required for adequate sensitivity and the inter-particle spacing required to give commercially acceptable feed rates. This effect is further compounded by the additional effects of particles (P-2) and (P+2), but these are second order effects and may be ignored.
For example, in practice, for 37 mm particles, a particle (P+1) of grade 0.5 gm/ton preceding a 37 mm waste particle (P) with a spacing of 100 mm will result in the particle (P) being seen as 0.12 kgm/ton and for an accept machine setting of 0.1 kgm/ton consequently being spuriously accepted. This ignores the additional effect of a following ore particle which may further increase the apparent grade of the particle (P). This effect increases rapidly with larger particles and smaller separations.